Fiber Reinforced Plastic (FRP) is used in automotive applications to increase the rigidity and strength of a part while simultaneously reducing its weight. FRP is commonly used for adding strength and rigidity to a preshaped foam core.
FRP has also been used in the manufacture of parts having a hollow core. Hollow core application of FRP involves lining the interior of a mold cavity with fibrous material and placing an inflatable bladder within the mold. The bladder is inflated to form an internal wall surface and a resin injected into the fibrous material. The bladder is either left inside the FRP part or removed. Such a method is described in U.S. Pat. No. 4,863,771, issued Sep. 5, 1989. This method requires a new bladder for each part manufactured and does not provide an integral bond between the inflatable bladder and fibrous material. The inflatable bladder used is made of flexible material which does not add to the strength or rigidity of the final part.
It is also known to blow-mold thermoplastic bottles and containers having an exterior netting reinforcement. For example in U.S. Pat. No. 4,122,142, issued Oct. 24, 1978, a plastic parison was blow-molded to slightly smaller than its desired shape. A tubular netting material was overlaid the still pliable parison. The parison was further blow-molded to its final shape causing the netting reinforcement to partially embed in the parison's exterior surface. This process was primarily designed for thin walled beverage containers and is not suitable for manufacturing structural parts which add strength and rigidity to the article they are made a part of. The reinforcing netting does not maintain the structural integrity of the container when exposed to high temperatures.
It is also known to blow-mold a hollow body having an open-celled sponge material for use as a fuel tank. German patent 2,304,543, issued August 1974, discloses an extruded parison which contains a compressed sponge. The mold die pieces close and parison is inflated to take the shape of the mold cavity. The sponge is allowed to expand and fill the hollow space within the molded tank.